(1) Field of the Invention
The present invention relates to the solid oxide fuel cells and the production thereof.
(2) Related Art Statement
Since the solid oxide fuel cells (SOFCs) operate at high temperatures of around 1,000, their electrode reaction is extremely active, and a catalyst such as a noble metal such as precious platinum needs not be used at all. Further, since polarization is low and output voltage is relatively high, energy-converting efficiency is far greater as compared with fuel cells of other types. In addition, since constituent materials of the SOFC are all solid, the SOFC is stable, and has a long service life.
The solid electrolyte film is generally formed by the dry process or the wet process. As typical dry processes, an EVD process and a spraying process may be recited. As the wet process, a tape casting process, a slip casting process, and an extruding process may be recited (Energy General Engineering No.13-2, 1990).
If the gas phase process such as a chemical vapor deposition (CVD) process or an electrochemical vapor deposition (EVD) process is employed, the size of a film-forming device becomes larger, and a treatable area and a treating speed are too small. Furthermore, since zirconium chloride or the like is used or steam is used in a mixed state with oxygen, running cost becomes higher.
If the solid electrolyte film is formed by plasma spraying, the film-forming speed can be made greater and handling of the film-forming device is easy. In addition, the thin film can be relatively densely formed. For this reason, the plasma spraying technique has been employed in recent years (Sunshine 1981, Vol. 2, No. 1, Energy General Engineering 13-2, 1990).
However, porosity of the solid electrolyte film formed by the plasma spraying is generally more than 5%, and may be up to 10%. Consequently, such a film has insufficient density as a solid electrolyte film for the SOFC, and therefore, cracks or a stratified defect occur in the film during plasma spraying. Owing to this, a fuel such as hydrogen or carbon monoxide penetrates the solid electrolyte film during operation of the SOFC, so that an electromotive force per SOFC becomes smaller as compared with a case where no such fuel penetration occurs. Consequently, the output of the SOFC drops and the rate for converting the fuel to power decreases.
Furthermore, fuel electrodes and air electrodes of adjacent cell units of the SOFC are generally connected to each other in series through an interconnector and a connecting terminal. Therefore, it is desired that the interconnector is made thinner to reduce its electric resistance. On the other hand, since the interconnector functions to separate the oxidizing agent from the fuel, gas-tightness is required.